In general, a compression refrigerator include at least a compressor, a condenser, an expansion mechanism (such as an expansion valve), and an evaporator, and further a drier, and is structured so that a mixed liquid of a refrigerant and lubricating oil (refrigerator oil) circulates in the closed system. In such compression refrigerator, the temperature in the compressor is generally high, and the temperature in the condenser is generally low, though such general theory is not applicable to a certain kind of such compression refrigerator. Accordingly, the refrigerant and the lubricating oil must circulate in the system without undergoing phase separation in a wide temperature range from low temperature to high temperature. In general, the refrigerant and the lubricating oil have regions where they undergo phase separation at low temperature and high temperature. Moreover, the highest temperature of the region where the refrigerant and the lubricating oil undergo phase separation at low temperature is preferably −10° C. or lower, or particularly preferably −20° C. or lower. On the other hand, the lowest temperature of the region where the refrigerant and the lubricating oil undergo phase separation at high temperature is preferably 30° C. or higher, or particularly preferably 40° C. or higher. The occurrence of the phase separation during the operation of the refrigerator adversely affects the lifetime or efficiency of the refrigerator to a remarkable extent. For example, when the phase separation of the refrigerant and the lubricating oil occurs in the compressor portion, a movable part is insufficiently lubricated, with the result that baking or the like occurs to shorten the lifetime of the refrigerator remarkably. On the other hand, when the phase separation occurs in the evaporator, the lubricating oil having a high viscosity is present, with the result that the efficiency of heat exchange reduces.
A chlorofluorocarbon (CFC), a hydrochlorofluorocarbon (HCFC), or the like has been heretofore mainly used as a refrigerant for a refrigerator. However, such compounds each contain chlorine that is responsible for environmental issues, so investigation has been conducted on a chlorine-free alternative refrigerant such as a hydrofluorocarbon (HFC). A hydrofluorocarbon typified by, for example, 1,1,1,2-tetrafluoroethane, difluoromethane, pentafluoroethane, or 1,1,1-trifluoroethane (hereinafter referred to as “R134a”, “R32”, “R125”, or “R143a”, respectively) has been attracting attention, and, for example, R134a has been used in a car air conditioner system.
However, the HFC may also be involved in global warming, so the so-called natural refrigerant such as carbon dioxide, a mixed refrigerant of fluoroiodomethane and propene, ether, or the like has been attracting attention as an alternative refrigerant additionally suitable for environmental protection.
Further, in recent years, an unsaturated fluorinated hydrocarbon compound (see, for example, Patent Document 1), a fluorinated ether compound (see, for example, Patent Document 2), a fluorinated alcohol compound, a fluorinated ketone compound, or the like has been found to be a refrigerant which: has a global warming potential lower than that of R134a described above; and can be used in a current car air conditioner system.
By the way, in the field of air conditioning, investigation has been recently conducted on a reduction in viscosity of refrigerator oil or an improvement in frictional characteristic of the oil in lubrication with a view to saving energy consumed by a refrigerator.
The energy-saving property of, for example, a refrigerator for a cold storage has been improved by reducing the viscosity of refrigerator oil to VG32, 22, 15, or 10. However, an additional reduction in viscosity has involved the emergence of problems such as reductions in sealing property and lubricity of the oil.
For example, (1) a refrigerator oil composition obtained by blending base oil composed of synthetic oil with at least one kind which: is selected from (A) an etherified product of a trivalent to hexavalent aliphatic polyhydric alcohol and (B) an etherified product of a bimolecular or termolecular condensate of a trivalent to hexavalent aliphatic polyhydric alcohol; and has a kinematic viscosity of 5 to 200 mm2/s at 40° C. (see, for example, Patent Document 3), and (2) a refrigerator oil composition obtained by blending base oil composed of mineral oil and/or synthetic oil with a glyceryl ether compound represented by the following general formula (I) at 0.01 to 10% by wt with reference to the total amount of the composition (see, for example, Patent Document 4) have been disclosed as refrigerator oil compositions each of which: is used in a compression refrigerator in which, for example, a hydrofluorocarbon-based, ether-based, hydrocarbon-based, carbon dioxide-based, or ammonia-based natural refrigerant is used as a refrigerant; and has improved lubricating performance:R1—OCH2CH(OH)CH2OH  (I)where R1 represents an alkyl group having 10 to 22 carbon atoms.
The refrigerator oil composition (1) has been investigated while emphasis is placed mainly on abrasion resistance and the property with which the clogging of a capillary is prevented, and the refrigerator oil composition (2) has been investigated while emphasis is placed mainly on abrasion resistance. However, no investigation has been conducted on the energy-saving property of each of the compositions.
Patent Document 1: JP 2006-503961 A
Patent Document 2: JP 7-507342 A
Patent Document 3: JP 10-265790 A
Patent Document 4: JP 11-315295 A